Here's a second stab at the spool hangar. It follows the same theory as the first one (using the front slat as a fulcrum and the back slat for support), but where the original just hooked over the front slat (putting all the stress on that small hook), this one is much more solid, resting atop the slat. 

This also has the same potential weakness as the first entry, though: it relies on the weight of the other spools (and the "grip" of the brackets) holding the rear slat in place. I don't see this as much of an issue, but if you try to use it to hold a really heavy spool on an empty rack, you might run into a problem.

As before, this is a big print, so I can't print it myself, but I've attached renders, and it should fit on the GMax.